Focused laser beams are used for locally treating or processing materials. In this connection, what is it is particularly important in the processing of a material is the spatial distribution of the light intensity in the focus of the laser beam. This spatial distribution may be described by iso-intensity surfaces, also referred to as isophotes. The shape and size of the isophotes are determined by the focusing optics and the indices of optical refraction of the materials to be irradiated, and usually they have the shape of an ellipsoid of revolution whose longest axis points in an axial direction with respect to the path of the laser beam.
When a laser beam is focused into a material that is transparent to the wavelength used, the light intensity within a small volume at the focus of the laser beam may exceed the material-specific threshold for multi-photon absorption. The volume within which this absorption takes place is defined by the profile of the isophotes. Local absorption in this volume may cause physical or chemical changes which, for example, in the case of irradiation of photoresists, results in selective chemical solubility in subsequent treatment steps.
In Nature 412, pp. 6976-698 (2001), S. Kawata, H. B. Sun, T. Tanaka and K. Takada describe what is known as “direct laser writing”. In this technique, a photosensitive material is irradiated by a laser whose frequency is below the single-photon polymerization threshold of the photosensitive material. When this laser is focused into the material, the light intensity within a small volume located at the focus and defined by isophotes may exceed the threshold for multi-photon polymerization. Here, too, this volume typically has the shape of an ellipsoid of revolution whose longest axis points in an axial direction. This type of irradiation produces physical or chemical changes in the material exposed to the laser beam.
M. Martinez-Corral, C. Ibáñez-López, G. Saavedra and M. T. Caballero, Optics Express, 11, pp. 1740-45 (2003) and C. Ibáñez-López, G. Saavedra, G. Boyer and M. Martínez-Corral, Optics Express, 13, pp. 6168-6174 (2005), describe amplitude masks which include a ring having lower transmission than the regions surrounding the ring and which may be used in microscopy, in particular, two-photon scanning microscopy, fluorescence microscopy, and confocal microscopy. Here, the amplitude masks are used to improve resolution during passive collection of light.
U.S. Patent Publications US 2006/0171846 A1, US 2005/0046818 A1 and U.S. Pat. No. 6,618,174 B2 all describe optical arrangements which serve as spatial filters and include a laser. A focusing element is placed between the laser and an amplitude mask in the path of the laser beam emerging from the laser in such a manner that the laser beam is first focused in the focusing element before it then hits the amplitude mask. Consequently, the amplitude mask is not in the collimated beam emerging from the laser.
German Patent Application DE 10 2004 013 886 A1 describes an optical arrangement which includes a laser and is used as a projection mask. Here, too, a focusing element is placed between the laser and an amplitude mask in the path of the laser beam emerging from the laser in such a manner that the laser beam is first focused in the focusing element before it hits the amplitude mask and, thus, the amplitude mask is not in the collimated beam emerging from the laser.
German Patent Application DE 10 2005 009 188 A1 describes an optical arrangement including a laser, where a first amplitude mask is placed between the laser and a focusing element in the path of the laser beam emerging from the laser, the laser beam first hitting said first amplitude mask. The first amplitude mask is used to influence the amplitude of the beam profile at a beam splitter. This beam splitter may have an annular configuration designed to minimize losses during fluorescence detection. Further, a second amplitude mask is used to additionally provide for spatial filtering, for example, to suppress disturbing diffraction orders, also at the beam splitter.